1. Field of the Invention
The present invention relates to a magnetic antenna and an antenna device used for radio frequency identification (RFID) systems, etc., that communicate with external apparatuses via electromagnetic signals.
2. Description of the Related Art
Mobile electronic apparatuses, such as cellular phones and data readers/writers, used in increasingly popular RFID systems, communicate with each other using respective antennas provided therein for information communication.
In a mobile electronic apparatus, in particular, there is a strong demand for a high performance, low cost and smaller size antenna. As an antenna that satisfies such requirements, a magnetic antenna having a magnetic core is disclosed in Japanese Unexamined Patent Application Publication No. 2002-325013.
Referring to FIG. 1, which shows a plan view of a magnetic antenna disclosed in Japanese Unexamined Patent Application Publication No. 2002-325013, an antenna coil 10 includes an air-core coil 12 formed by winding a conductor (11a, 11b, 11e, 11d) in a spiral shape in a plane on a film 12a and a flat magnetic core member 13 inserted in the air-core coil 12 so as to be substantially in parallel with the plane of the air-core coil 12. The air-core coil 12 has a hole 12d into which the magnetic core member 13 is inserted. A first terminal 11a and a linking conductor 11e are connected with a through hole 12b, and a second terminal 11b and the linking conductor 11e are connected with a through hole 12c. This magnetic antenna is arranged on a conductor plate 14.
The magnetic antenna shown in FIG. 1 disclosed in Japanese Unexamined Patent Application Publication No. 2002-325013 has a metal plate at the backside thereof. Magnetic flux passes in a substantially horizontal direction from right to left in the state shown in FIG. 1, whereby electromotive force is generated, and a current flows in the coil conductor.
However, when two of the magnetic antennas shown in FIG. 1 are provided to configure a pair of magnetic antennas by connecting the coil conductors thereof such that the currents flowing in the two coil conductors have the same phase, problems are encountered, such as described below with reference to FIGS. 1, 2A, and 2B.
FIGS. 2A and 2B show two kinds of configurations in which a pair of the magnetic antennas shown in FIG. 1 are provided.
When the magnetic antennas shown in FIG. 1 are to be configured to form a pair, two coil conductors are formed on one flexible substrate, and a magnetic core 13 is inserted into each of the winding centers of the two coil conductors formed on the films 12a, as shown in FIGS. 2A and 2B, to facilitate easier handling and lower cost of the parts.
FIG. 2A shows an example in which portions of the films 12a that connect the respective winding portions of the two coil conductors, into which the magnetic cores 13 are inserted, face the conductor plate 14. In this structural arrangement, if the conductor plate 14 is a circuit substrate, for example, then other parts cannot be mounted in an area of the circuit substrate facing the films 12a, thus causing a lower packaging density of the portion where the magnetic antenna is mounted.
FIG. 2B shows an example in which portions of the films 12a that connect the respective winding portions of the two coil conductors, into which the magnetic cores 13 are inserted, are arranged in such a manner as to be apart from the conductor plate 14. In this mounting structure, the coil conductors formed on the films 12a are close to the conductor plate 14. Hence, the magnetic flux passing through the winding centers of the coil conductors is limited to magnetic flux that enters the magnetic cores 13 through the end portions thereof, thus causing a decrease in antenna performance (i.e., a decrease in gain).